Yay, new playpen!
Rats are first and foremost xenophobes. If you got them from a pet store, they are also traumatized xenophobes who've never been handled in their lives.
My rats were bred for friendliness and handled their entire lives from birth. They /still/ approach things cautiously the first 12 or more times. It will likely take your fearful kids weeks of regular exposure or more to learn trust.
Here's a link to one of my threads where my sweet girls met a new human for the first time just to give you some idea of how they approach new situations. These are well socialized girls, and it still took them 20 minutes to decide the new kid wouldn't eat them.
You could put them in your sweater and spend time doing 'forced socialization'...which to be successful requires a sensitivity to their needs and knowing exactly when to back off and when to push things. Depending on their personality, it may even speed things along and get them to bond with you more quickly. Also depending on their personality, it could cause them to be more distrustful and take much longer to bond with you.
However, patience will always yield better results. If you can set their play area up in such a way that you can stretch out and hang out reading a book or studying, you'll be much more comfortable, and they can explore you at their leisure.
Something you can do also, is close their cage while they are out to prevent them from using it as a hide away. Then provide a safe alternate bolt hole for them outside their cage. Even better, setting up the bolt hole/hiding place for them incorporating your body. For example, cover yourself in a blanket while you study, leaving lots of caves and hidey hole folds in it. If the blanket is the only real option they have for finding safe cover, they'll use it and then gradually come to associate your scent and warmth with their new 'safe zone'.
You aren't forcing them to come to you, you're just making yourself more attractive option and blackmailing them with their own natural instincts. Hiding fragments of favorite treats in and around your person as part of the process works too.
If you're totally ignoring them during each exploration process...doing your homework or reading while they do this is a good way for you to not be over excited or stressed with them ignoring you...your body language and scent will be more relaxed, and they will be too.
Never move fast towards them, and never grab for them. If you need to put them away, just open the cage door and put food in there. They'll run for it soon enough as that is their home.
There's tons of socialization stuff out there. Some of it conflicting and none of it meant to be a one size fits every rat or rat owner. I'll be happy to share with you some of what I've found.
But it will all basically be variations of 'take your time, use their natural instincts to your advantage, pay attention to their body language, and let them come to you if possible, but sometimes you will need to take them outside their comfort zone in gentle stages too'.